1. Field of the Invention
The present invention relates to a raindrop detecting device and a method of determining a raindrop amount.
2. Description of Related Art
A raindrop detecting device typically includes a raindrop sensor, which is an optical sensor having a light-emitting element and a light-receiving element. The light-emitting element emits light, and the emitted light is reflected by a surface of a front windshield. The light-receiving element is located such that the reflected light can be received by the light-receiving element.
When any raindrop does not exist in a detection area of the raindrop sensor, the front windshield reflects almost all the emitted light and the reflected light is received by the light-receiving element. In contrast, when raindrop exists in the detection area of the raindrop sensor, not all of the emitted light is reflected by the front windshield. That is, a part of the emitted light travels outside of the front windshield, due to the raindrop. Therefore, an amount of light incident into the light-receiving element is reduced, compared with a case in which any raindrop does not exist. When the amount of light incident into the light-receiving element is reduced, a signal output from the light-receiving element is lowered. When the lowering of the signal output from the light-receiving element is detected, a raindrop amount can be determined to increase. That is, the raindrop amount is determined based on the signal output from the light-receiving element. Therefore, a wiper can be controlled based on the determined raindrop amount.
Here, JP-A-2002-46580 discloses a raindrop detecting device. When a wiper is activated such that a wiper blade wipes raindrop from a front windshield, the raindrop wiped by the wiper blade is collected in a moving direction of the wiper blade. That is, the raindrop moves along a surface of the front windshield together with the wiper blade. Because a raindrop sensor of the raindrop detecting device is located in a wiping area of the wiper blade, the collected raindrop passes through a detection area of the raindrop sensor, when the wiper blade passes through the detection area of the raindrop sensor. That is, an amount of water passing through the detection area of the raindrop sensor becomes large due to the collected raindrop, compared with an amount of raindrop regarding rainfall.
In this case, an amount of light incident into the light-receiving element is much reduced, so that the raindrop detecting device may erroneously determine that it is raining heavily. In order to prevent this erroneous determination, the raindrop detecting device is prohibited from performing the determination of the raindrop amount for a predetermined period while the wiper is operating. The predetermined period represents a period that the wiper blade passes through the detection area of the raindrop sensor and the adjacent area.
The predetermined period is set longer such that the wiper blade secures to be located outside of the detection area of the raindrop sensor when the determination is to be performed. That is, the predetermined period is set by adding a leeway period to a period that the wiper blade is located inside of the detection area of the raindrop sensor.
The period that the wiper blade is located inside of the detection area of the raindrop sensor can be calculated based on a position of the raindrop sensor and a moving speed of the wiper blade. The leeway period is to be set in consideration of a variation of the moving speed of the wiper blade. For example, a manufacturing variation of the wiper blade, a wear of the wiper blade, or a variation of a friction coefficient between the wiper blade and a glass constructing the front windshield causes the variation of the moving speed of the wiper blade. Because the leeway period has to be set by taking these variation factors into consideration, the leeway period becomes long, so that a period allowing to perform the determination of the raindrop amount becomes short. In this case, accuracy for determining the raindrop amount may be lowered.
Further, a timing and a length of the predetermined period depends on a position relationship between a wiping area of the wiper blade and the raindrop sensor. Therefore, the timing and the length of the predetermined period are required to be set relative to each vehicle type. That is, a specific controller, e.g., microcomputer, may be needed relative to each vehicle type.